Not applicable.
1. Field of the Invention
The present invention relates generally to the field of vertical cavity surface emitting laser arrays. More specifically, it relates to vertical cavity surface emitting laser arrays that emit light at different wavelengths, and to a method of producing such arrays binary masks.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. In a VCSEL, optical emission occurs normal to the plane of a PN junction. VCSELs have certain advantages over edge-emitting laser diodes, including smaller optical beam divergence and well-defined, highly circular laser beams. Such advantages make VCSELs well suited for optical data storage, data and telecommunication systems, and laser scanning.
VCSELs can be formed from a wide range of material systems to produce specific characteristics. VCSELs typically have active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or by using molecular beam epitaxy (MBE).
To assist the understanding of VCSELs, FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) substrate 12 is disposed with an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAS substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20 having a plurality of quantum wells is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that is formed by implanting protons into the top mirror stack 24 or by forming an oxide layer. In either event, the insulating region 40 has a conductive annular central opening 42 that forms an electrically conductive path though the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and its conductive central opening 42 confine the current 21 flow through the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, providing a p-type substrate), different material systems can be used, operational details can be varied, and additional structures, such as tunnel junctions, can be added. Furthermore, FIG. 1 only illustrates one VCSEL.
Producing multiple VCSELs on one substrate can be beneficial. In some applications, such as data and telecommunication systems, it is beneficial to have a VCSEL array that is comprised of multiple individual VCSEL elements that emit light at different wavelengths. Such an array could be used to implement wavelength division multiplexed systems. That is, light of one wavelength could be emitted (and, if required, modulated), then light of another wavelength could be emitted (and, if required, modulated), and so on. Because of the inherent low cost and volume capability of VCSELs, a VCSEL array suitable for wavelength division multiplexing would be highly attractive.
However, despite their many benefits, VCSEL arrays suitable for wavelength division multiplexing are not commercially available. One reason for this has been the unavailability of a low cost method of producing stable wavelength division multiplexed light beams from a single substrate.
In a VCSEL, the wavelength of the light output depends on various factors, one of which (as previously noted) is the separation of the top DBR mirror and the bottom DBR mirror. Thus, the output wavelength can be tuned by controlling the length of the cavity between the top and bottom DBRs. That cavity length is set during the manufacturing process. FIG. 2, which illustrates a side view of a simplified VCSEL element 98 of a VCSEL array, is useful for visualizing the cavity length. As shown, the VCSEL element 98 includes a substrate 100 having a backside contact 102 and a backside DBR mirror 104. An active region 106 is on the backside DBR mirror 104. A front side DBR 110 is on the active region 106. Front side electrical contacts 112 are on the front side DBR 110. Thus, the front side and back side DBR separation is controlled by the width of the active region 106 (and by the reflection depth of the DBRs). Therefore, the output wavelength is controlled by the processes that form the VCSEL element.
Therefore, a process of producing a VCSEL array that emits light beams of different wavelengths would be beneficial. Even more beneficial would be a new VCSEL array that is suitable for wavelength division multiplexing. Still more beneficial would be a low cost lithographic technique of producing VCSEL arrays that emit light beams having different wavelengths.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
Accordingly, the principles of the present invention are directed to a method of producing VCSEL arrays, and to VCSEL arrays produced by that method, that are capable of emitting light beams having different wavelengths and that are suitable implementing wavelength division multiplexing in a cost effective manner. According to the principles of the present invention, binary masks are used to control depositions and/or etchings of a spacer that is disposed between top DBR mirrors and an active region. By using the binary masks, the wavelengths of individual VCSEL elements on a common substrate can be controlled.
According to one method that is in accord with the principles of the present invention, a process-controlled spacer is selectively grown on an active region using a sequence of binary masks such that the spacer has multiple thicknesses that are controlled by the binary masks. Then, front side (top) DBR mirrors are disposed over the spacer. Electrical contacts for the individual VCSEL elements are then provided. Additionally, suitable isolation regions are formed, either in the spacer or in the front side DBR mirrors, such that discrete VCSEL elements are formed. Suitable spacers can be formed from regrowth AlxGa(1xe2x88x92X)As (or similar materials), a dielectric deposition (such as PECVD SiO2), or a glass deposition.
According to another method that is in accord with the principles of the present invention, a process-controlled spacer is formed over an active region. Then, that process-controlled spacer is selectively etched using a sequence of binary masks such that the spacer has multiple thicknesses in locations controlled by the binary masks. Beneficially, the spacer includes etch stop layers that accurately control the etch depth, and thus the spacer thicknesses. Then, front side DBR mirrors are disposed over the spacer and electrical contacts for the individual VCSEL elements are provided. Additionally, isolation regions are formed, either in the substrate or in the front side DBR mirrors, such that discrete VCSEL elements are produced. Suitable etch processes include isotropic planar etching, with the particular etchant that is used being dependent on the spacer material.
A VCSEL array according to the principles of the present invention includes a substrate, an active region adjacent the substrate, and a spacer having a plurality of regions with different thicknesses. Beneficially, the difference in thickness between each region is a multiple of a distance L. Front side DBR mirror structures are over the spacer, and electrical contacts for the individual VCSEL elements are over the front side DBR mirror structures.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.